1. Field of the Invention
The invention relates to improvements in electrostatic printing, more particularly to electrostatic printing plates with a tiered surface which yields superior results in part by providing a mechanical spacing between toner layer and receiving surfaces.
2. Description of the Related Art
The modern electrostatic printing plate was first described by Reisenfeld (U.S. Pat. No. 4,732,831). These plates can be used with both liquid and dry toners; however, in practice, many likely plate materials are very soft and tacky and cannot be used with dry toner for this reason. The use of durable and non-stick overcoats on printing plates, described by Detig (U.S. Pat. Nos. 4,859,557 and 5,011,758) has allowed such plate materials to be used with dry toner.
If one is using liquid toners and/or one is printing on electrically conducting surfaces (like metal) a well defined mechanical gap between the printing plate surface and the surface to be printed must be maintained. The gap prevents the receiving surface from crushing the toner layer and deforming it mechanically; also, if a metal surface touches the printing plate the transfer voltage will cause electrical breakdown of the printing plate in areas of weak dielectric strength.
Maintaining these xe2x80x9cwell-definedxe2x80x9d mechanical gaps using current printing methods is not simple and in some cases impossible. When attempting to print onto xe2x80x9cflatxe2x80x9d glass it is found that such materials have both short range and long range thickness and flatness variations, which complicates the maintenance of the required mechanical gaps. In addition, the use of elastomeric toner transfer rollers to solve such problems causes placement errors in the transfer image, which are not tolerable in the manufacture of high-resolution objects, such as flat panel display, screens. The overlay accuracy between one layer and the next layer beneath it is lost.
In any large apparatus, like a cylindrical printing drum impinging on a rigid non-flat surface like glass or a metal plate; there is no true mechanical contact between the two objects except for a few high spots. This is the reason that gap transfer of toner is an important technology.
The transfer of liquid toners across mechanical gaps was described by Bujese of Olin Hunt in the patent literature. In these teachings the printing surface, either an electrostatic printing plate or photo receptor plate like in a laser printer; the mechanical gap between plate and receiving surface must be maintained by external means. This is often difficult to accomplish as the dimensions of the drum and plate approach the order of xc2xd to 1 meter or larger.
The present invention addresses and overcomes the problem of current printing methods by providing the required mechanical gaps via a tiered printing plate which maintains the mechanical gap even on uneven surfaces while still retaining the ability to precisely locate the plate and the printed image.